In the past, a variety of magnetic materials and structures have been utilized to form magnetoresistive materials for non-volatile memory elements, read/write heads for disk drives, and other magnetic type applications. One prior magnetoresistive element utilized a magnetoresistive material that has two magnetic layers separated by a conductor layer. The magnetization vectors of the two magnetic layers typically are anti-parallel to each other in the absence of any magnetic fields. The magnetization vectors of one of the layers points in one direction and the magnetization vector of the other layer always points in the opposite direction. The magnetic characteristics of such magnetic materials typically require a width greater than one micron in order to maintain the orientation of the magnetization vectors along the width of the cell. The large width requirement limits the density of memories utilizing such materials.
Another type of memory cell uses multi-layer giant magnetoresistive materials (GMR) and utilizes dimensions around one micron, in order to increase density. A conductive layer is again disposed between the multi-layers of magnetic material. In this structure the magnetization vectors are parallel to the length of the magnetic material instead of the width. In one embodiment the magnetization vector of one magnetic layer is always maintained in one direction while the magnetization vector of the second magnetic layer switches between parallel and antiparallel to the first vector in order to represent both logical "0" and "1" states. This structure is commonly referred to as a spin valve structure. In another embodiment the magnetization vectors of both magnetic material layers are always maintained in the same direction, with opposite vector directions representing logical "0" and "1" states.
Still another type of memory cell uses multi-layer giant magnetoresistive materials (GMR) and also utilizes dimensions around one micron, in order to increase density. In this type of cell a non-conductive layer is disposed between the multi-layers of magnetic material. The magnetization vectors are again parallel to the length of the magnetic material instead of the width but sense current tunnels through the non-conducting layer from one layer of magnetic material to the other, rather than being conducted lengthwise. This structure is commonly referred to as a tunneling GMR cell.
A magnetic random access memory (MRAM) is a non-volatile memory which basically includes a GMR cell, a sense line, and a word line. The MRAM employs the GMR effect to store memory states. Magnetic vectors in one or all of the layers of GMR material are switched very quickly from one direction to an opposite direction when a magnetic field is applied to the GMR cell over a certain threshold. According to the direction of the magnetic vectors in the GMR cell, states are stored, and the GMR cell maintains these states even without a magnetic field being applied. The states stored in the GMR cell can be read by passing a sense current through the cell in a sense line and sensing the difference between the resistances (GMR ratio) when one or both of the magnetic vectors switch. The problem is that in most GMR cells the GMR ratio is relatively low (e.g. 10% or less) and, consequently, reading or sensing the state stored in the GMR cell can be relatively difficult. Some attempts have been made to increase the GMR ratio but most of these attempts result in an increase in the amount of current, or magnetic field, required to switch states. This increase in current, or magnetic field, can result in a substantial operating power increase, especially in large arrays of GMR cells.
Accordingly, it is highly desirable to provide magnetic random access memories and memory cells with increased GMR ratios and without increasing the magnetic field required for writing and reading.
It is a purpose of the present invention to provide a new and improved magnetic memory cell with increased GMR ratio.
It is another purpose of the present invention to provide a new and improved magnetic memory cell including an increased GMR ratio without changing the amount of magnetic field required for switching states.
It is a purpose of the present invention to provide a new and improved magnetic memory cell with increased GMR ratio with faster speed and low power consumption.